Computed radiography (CR) systems using stimulable phosphors enjoy broad acceptance as clinical imaging tools. In a CR system, a stimulable phosphor is exposed to an image-wise pattern of short wavelength radiation, such as X-ray radiation, to record a latent image in the stimulable phosphor. The latent image is read out by stimulating the phosphor with stimulating radiation of a first wavelength, such as red or infrared light. Upon stimulation, the stimulable phosphor emits stimulated radiation of a second wavelength, such as blue or violet light, representative of the latent radiographic image. To produce a signal useful in digital image processing, in one well known flying spot technique, the stimulable phosphor is scanned in a raster pattern, by a beam of light, produced for example, by a laser reflected by an oscillating or rotating mirror. The stimulated radiation from the phosphor is sensed by a photodetector, such as one or more photomultiplier tubes, to produce electronic image signals. (See: U.S. Pat. No. 5,105,079 (Boutet et al.)). Although suitable for the applications intended, such image readout systems are large and complex and produce only a single point of image data at a time.
More recent CR systems have improved upon this earlier technique by providing a full line of image data at a time, offering advantages of faster throughput and lower cost and complexity over flying spot scanners. As just one example, U.S. Pat. No. 6,373,074 B1 (Mueller et al.) is directed to a CR system that scans a full line of image data points at a time. FIG. 1 shows the basic components of such a system. As shown, system 10 includes a linear light source 12, typically using an array of laser diodes or other light sources, which directs a linear scanning beam 14 onto a stimulable phosphor sheet 16 that has been irradiated and stores a latent X-ray image. One or more cylindrical lenses 18 are used to direct the highly asymmetric linear output beam along a line 20 on the surface of phosphor sheet 16. In a sensing head 22, collection optics 24 then directs the stimulated light from line 20 on phosphor sheet 16 through an optical filter 26 and to a linear photodetector array 28, typically a charge-coupled device (CCD) array. Phosphor sheet 16 is indexed in direction D by a transport mechanism (not shown) to provide a page scanning motion. In this way, phosphor sheet 16 is moved past sensing head 22 to detect each line of the image stored thereon. The sensed image data is then processed by an image processor 30 that assembles a two-dimensional output image from each successive sensed line. The output image can then be stored, transmitted to another location, or displayed.
While there have been numerous improvements to apparatus and methods for obtaining the stored image on a stimulable phosphor, there is still a need for increased efficiency and overall image quality. One widely recognized problem with existing CR readers relates to the need for improved image quality at image sensing circuitry (generally represented as linear photodetector array 28 in FIG. 1). The apparatus disclosed in US Patent Publication Nos. 2002/0096653 (Karasawa); 2001/0028047 (Isoda); 2002/0040972 (Arakawa), and in the above referenced U.S. Pat. No. 6,373,074, and elsewhere, for example, employ Selfoc™ lenses and provide 1:1 imaging. While this solution allows compact packaging of the sensing components and their support optics, it imposes a constraint on numerical aperture (NA). The Selfoc™ gradient index lens is characterized as having a low NA. The maximum f/# value for this type of lens is typically about f/2, which provides an NA of 0.25. Because collection efficiency of this lens is proportional to the square of the NA value, a low NA can significantly degrade overall system brightness. Yet another disadvantage of existing systems relates to the relatively low fill factor of the Selfoc™ lens array. Gaps between adjacent Selfoc™ lens elements limit the fill factor and further constrain light collection.
As a result of the overall inefficiency of the collection optics, the signal-to-noise ratio (SNR) of conventional sensing systems is disappointing. Collecting light over a broader area, such as disclosed in US Patent Application Publication No. 2001/0028047 noted above, tends to further degrade the SNR relationship, even when using two-channel sensing optics. Low collection efficiency also constrains the reading speed of the stimulable phosphor reader.
The photolithography system disclosed in the following patents is also of interest: U.S. Pat. No. 4,391,494 (Hershel), U.S. Pat. No. 6,813,098 B2 (Mercado), U.S. Pat. No. 6,863,403 B2 (Mercado et al.), US Patent Application Publication 2004/0125352 A1 (Mercado). The optical system disclosed in these patents is illustrated in FIG. 2. Optical system 38 includes a concave spherical mirror 40, an aperture stop AS1 located at the mirror, and a composite, achromatic plano-convex doublet lens-prism assembly 42. Mirror 40 and assembly 42 are disposed symmetrically about an optical axis 44. Optical system 38 is essentially symmetrical to aperture stop AS1 located at mirror 40 so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system 38 are nearly concentric. In optical system 38, doublet-prism assembly 42 includes a meniscus lens 43A, a plano-convex lens 43B and symmetric fold prisms 45A and 45B. In conjunction with mirror 40, assembly 42 corrects the remaining optical aberrations. Symmetric fold prisms 45A and 45B are used to attain sufficient working space for movement of a reticle 46 and a wafer 48 located at respective object plane OP1 and image plane IP1. A beam of light 49 is transmitted through reticle 46 and the reticle image is transmitted by optical assembly 42 and mirror 40 to wafer 48.
The optical system discussed above has the following characteristics: (1) In order to minimize the aberration of the system, the system is large and heavy; (2) The objective of the system requires transmissive illumination; (3) The illumination light and the light collected by the system are the same, i. e., have the same wavelength. The latter characteristics limit this system's application to a stimulable phosphor radiation readout system because of the following requirements of the latter system: (1) Stimulable phosphor radiation readout requires reflection illumination; (2) Readout of the stimulable phosphor is carried out by stimulating radiation of a wavelength different than the wavelength of the readout stimulated radiation; (3) The stimulable phosphor readout system requires the filtering of the stimulating radiation from the photodetection system; (4) The stimulable phosphor readout system should be compact and lightweight.
The following patents disclose various optical systems that are of interest. U.S. Pat. No. 2,742,817 (Altman); U.S. Pat. No. 3,910,684 (Glatzel); U.S. Pat. No. 4,171,871 (Dill et al.)
There is thus a need for a radiation readout apparatus that is simple and compact, that has high image quality, that has high collection efficiency, and that has a large field of view.